Ground responsive protective system



Oct. 26, 1965 MOSER ETAL 3,214,638

GROUND RESPONSIVE PROTECTIVE SYSTEM Filed Sept. 26, 1960 United States Patent 3,214,638 GROUND RESPONSIVE PROTECTIVE SYSTEM Robert Moser, 9 Rue du Lycee Lakanal, Bourg-la-Reine, France, and Jacques Morel, 12 Rue Rennequin, Paris, France Filed Sept. 26, 1960, Ser. No. 58,427 Claims priority, application France, Oct. 2, 1959,

806,618 4 Claims. (Cl. 317-18) This invention prov-ides for protection against socalled leakage currents liable to produce electric shocks to personnel and it ensures simultaneously the automatic control of the condition of the circuits in electric ap paratus.

The risk of producing electric shocks to personnel is ascribable to various causes, so that the technical con ditions which ensure sufliciently, that is, through the release of a safety switch, the desired protection of persons, are of an extremely complex nature.

One of the main sources of such electric shocks which may frequently lead to electrocutions, is constituted by the leakage current passing through the human body between one phase line or a metal member which accidentally comes in contact with a phase line and ground; said current is the direct leakage current leading to ground through the human body. It should be remarked that a leakage current of the same intensity passing through a lead other than the human body will also cause the operation of the safety means. In certain unfavorable "cases, such a leakage current flowing through the human body which has a value not exceeding a few ten milliamperes may lead to electrocution.

It has already been proposed to insure the protection of persons against such a risk by resorting to switches controlled by the current flowing through the secondary of a transformer detecting the leakage current between a phase and ground and operating through the lack of equilibrium between the currents passing through the phase line and the neutral of the apparatus, which lack of equilibrium is ascribable to the leakage current. By reason of the low intensity of the dangerous current, the relays controlling such switches should be extremely sensitive.

However, numerous apparatus allow, through their actual structure, the formation of a normal leakage current passing through the grounding line, whereby a lack of equilibrium similar to that described hereinabove will normally exist. This is the case, in particular, of hot plates, furnaces, washing machines and the like. In other words, all. apparatus in which a transient current passing through the grounding line is grounded thereby as a result of insulation breakdown due to the possible presence of moisture, steam or condensed water. Such apparatus are provided of necessity, with a grounding line whose purpose is to prevent the voltage of the ground constituted by the frame of the apparatus from rising above a certain maximum, selected generally as equal to 24 volts; but, they may be brought to a higher and dangerous potential in the case of a direct contact between the phase line and ground, which requires opening of the switch for a leakage current, that is, a current constituted by the vectorial sum of the current flowing through the phase and the neutral line, which sum is diflerent from zero, and has the same intensity as in the case of an apparatus which is not provided with grounding means. This condition cannot be satisfied in practice, since the transient leakage or grounding current, allowed by the structure of the machine, has a value which may reach a multiple of the direct leaking current flowing to ground through the human body and which may be dangerous "ice for human beings. It has hitherto been the practice to employ a relay of the same type as that referred to heremabove but of a lower sensitivity, as provided by introducing into the grounding circuit a resistance, the value of which is selected so as to limit the voltage arising upon contact between the apparatus and ground to a value of which is selected so as to limit the voltage arising upon contact between the apparatus and ground to a value which is not dangerous for human beings. The protection is obtained, however, only when the continuity of the grounding line is ensured reliably.

Other causes of electric shocks to operating personnel and damage to electrical apparatus may be the result of a potential diiferent from zero for the neutral line of the power plant and, in particular, of said neutral potential rising above a predetermined value.

An object of this invention is therefore to provide a protective automatic circuit breaker for interrupting the power supplied to an electrical apparatus, by opening a safety switch if a predetermined value of any of the following parameters is exceeded. The invention distinguishes from conventional safety switches by the fact that it is controlled by at least two of the following parameters:

(1) The intensity of the leakage current through the .human body between one phase and ground, or between a grounded conductor which is accidentally in contact with a phase, and ground;

(2) The intensity of the normal leakage current provided by the structure of the apparatus and which flows through the grounding line;

(3) The potential of the neutral of the apparatus with reference to ground; and

(4) The ohmic resistance of the grounding line.

According to one aspect of the invention it relates to a safety switch forming a protection against electric currents, this switch being controlled by a relay energized by the secondary winding of a transformer. The transformer includes primary windings through which flow phase and neutral currents, thereby generating fluxes which are added as vectors. The sum of the fluxes thus added forms, for the maximum intensity allowable for a direct leakage current through the human body, a flux which is sufl'lcient for actuating the relay. Now, the present invention consists in allowing the passage of a normal leakage current through the grounding line of an intensity larger than that of the direct grounding current passing through the human body, as provided by the fact that the transformer feeding the relay includes an auxiliary primary winding connected in series with the grounding line having a number of turns which is different from that of the primary phase and neutral windings. This arrangement produces inside the transformer and for the maximum allowed value of the grounding current a compound flux which is equal to that obtained for the maximum value of the direct grounding current leaking through a human body.

This arrangement provides control of the intensity of the direct leakage current between a phase and ground through the human body, and control of any current produced by a defect in the apparatus. It also controls the intensity of the normally allowed leakage current which may flo-w through the grounding line in the operation of the apparatus considered.

According to a further feature of this invention, the control of the neutral voltage of the installation is insured by introducing a resistance connecting the neutral at a point between the apparatus to be protected and the primary winding fed by said neutral, with the grounding circuit, at a point between ground and the corresponding auxiliary primary winding. The value of said resistance is selected in a manner such that for a potential of the neutral rising above said predetermined value, the intensity of the current passing through the resistance towards ground as a leakage current flowing through the auxiliary winding may be equal to the maximum intensityallowed for the leak current passing through the human body, which causes the opening of the switch.

Furthermore, this arrangement insures at the same time control of the contact voltage between the metal frame of the apparatus and ground and also that of the ohmic resistance of the grounding line. This voltage, which may appear only if the ohmic resistance of the grounding line is abnormally high, produces the passage through said resistance of a leakage current flowing both through the primary winding of the neutral and through the auxiliary primary winding of the grounding line. I

For clarity, the wiring to only one phase of the supply current is shown, as understood by those skilled in the art.

The invention will now be described in further detail, reference being made to the accompanying drawings illustrating a preferred embodiment of said invention.

The single figure of said drawing is a wiring diagram of an automatic circuit breaker according to the invention. The safety device or automatic breaker shown enclosed within the broken lines is generally designated by the reference number 1.

The low voltage installation to be protected is connected between a phase line Ph and the neutral line N of an alternating current source of electrical power.

RN is .a grounding resistance for the neutral of the system, the value of which resistance may be equal, for instance, to 1 ohm. The power source feeds an electric machine or apparatus 2 such as a hot plate, a furnace, a washing machine or the like, the frame of appliance 2 is connected by line M groundedby T through a line in which is inserted a resistance RM of about 50 ohms. During normal operation, a leakage current produced by the presence of water, steam or condensed water on the electric components of the apparatus appears transiently in the grounding line and may reach several hundrcd milliamperes.

A further electric apparatus 3, which is not grounded, may also be energized by bridging it across the apparatus 2, which may be any conventional electrical appliance.

By means of the invention persons using, or otherwise coming into contact with such an arrangement are protected against any current arisingthr-ough electric shocks. These risks are ascribable to various causes. For example, in the case of a poor insulation of the leads, the phase line may come into contact at 4 with the frame of the apparatus 2 or of the apparatus 3, which contact may bring the frame of the apparatus 3 to a potential approximating the supply voltage. It might also cause a leakage current to flow into the grounding line T, such leakage current being larger than the normal leakage current due to water, for example in the apparatus 2.

The potential appearing thus in the frame of the apparatus 3. is dangerous, since it may lead to a leakage current passing into ground through the human body and rising above a maximum intensity Idh. The intensity Idh is the value of leakage current that will cause the automatic breaker to interrupt the power to the apparatus, and this value is preferably equal to 40 milliamperes as a maximum. It may, however, have a different value according to local safety requirements.

Considering the apparatus 2, the resistance of the grounding line should be 'sufiiciently low to prevent the frame of the apparatus from being raised to a so-called contact voltage which is such that there may appear a risk of electrocution. But, it is important to detect such leakage currents as soon as their intensity rises above a maximum intensity Idc, which is the maximum current to be tolerated as normal leakage current through allowed by the structure of the apparatus and which may be equal to 200 milliamperes, for instance, such a direct contact between a lead and the frame of the apparatus would be indicated if the leakage current were to exceed Idc.

Furthermore, the grounding line may become effectively open, circuited or broken as a consequence of an accidental increase in the ohmic value of the resistance RM or of a break in said grounding line. The frame of the apparatus 2 may then reach a potential which becomes dangerous when above a maximum value U0 Lastly, the neutral line N may be raised to an abnormal potential. The potential of the neutral line N should, in no case rise above a value U such as 22 volts.

To ensure such control operations, a transformer 5 produces the vectorial addition of the fluxes generated by primary windings 6. The different currents to be controlled are fed to these primary windings and are 'constituted by the current in the phase line or lines and by the neutral current. In the'circuit illustrated, which is of a one-phase type, only two windings are used, respectively, for the single phase and for the neutral.

The transformer 5 includes furthermore an auxiliary winding 7 connected between the frame M and the grounding line T in series with the latter. The secondary 8 of the transformer is connected in series with the winding 9 of the relay 10, the armature 11 of which, when drawn in, opens the safety switch or circuit breaker which comprises chiefly the contact pieces 12in the phase and neutral circuits. When the local safety regulations prohibit any breaking of the neutral, the contact piece 12 in the neutral line may be omitted. The safety switch may also be opened by hand by a knob 13 and it may be reengaged by a knob 14.

The circuit includes furthermore, a resistance 16 connected to the appliance winding between the neutral circuit at a point beyond primary winding 6 and connected to the grounding circuit at a point beyond the auxiliary primary winding 7 of the latter, that is, in the section of the grounding circuit leading to ground.

The operation of the automatic breaker of this invention is as follows: during normal service, the primary windings 6 carry currents of equal intensities producing fluxes, the vectorial sum of which is zero. Assuming there is no leakage current flowing to ground, no current flows through the secondary 8.

If, in contradistinction, there arises an accidental contact at 4 between the phase line and the frame of the apparatus 3, which is not grounded, and if a grounded human body then contacts the frame, the leakage current flowing then to ground will produce a lack of equilibrium between the fluxes passing through the two windings 6. When said current reaches a sufficient value defining the sensitivity of the relay, the electromagnet 10 draws in its armature 11 and opens the safety switch 12. The current sensitivity of the automatic breaker cannot 'be such that the intensity Idh of the leakage current should not rise, for instance, above 40 milliamperes without causing the switch 12 to be opened by relay 10. The sensitivity depends partly on the number of ampere-turns N of the windings 6.

The threshold for which the relay operates should therefore be reached for a number of ampere-turns NI equal to NIdh the value allowed for which is equal to 1,200 mNI which provides i240 milliamperes, and N:3O turns.

The result is that, for any leakage current passing through the frame of any auxiliary apparatus 3 with an intensity above Idh, the relay operates and opens the switch.

With the circuit described, any grounding leakage current arising in the frame of the apparatus 2 flows through the winding 7. 7

If said winding 7 includes N turns the flux produced thereby will be proportional to N i'. Said flux is added as a vector inside the transformer to the fluxes produced by the phase and neutral currents, which currents have a lack of equilibrium of their amperage corresponding to the leakage current which is equal to 1''. Therefore, the lack of equilibrium of the fluxes is equal to N, the sign of which is made opposite to that of .the flux N i' by reason of the circuit connections shown in the drawing.

The windings 6 are connected in series in the lines Ph and N respectively so as to give in the transformer coil an algebraic amount of fluxes induced equal to zero when the currents are balanced, that is, if said windings have the same direction of winding, they have their bounds on the same side connected to the phase and neutral terminals of the supply system and their bounds on the other side connected to the protected electrical power source.

The winding 7 must be connected in the conductor TM so as to induce in the coil a flux having the same direction as that induced by a current flowing in the winding 6 connected in the conductor N. That is to say, the connection is the same as above disclosed.

If the winding 7 has the same number of turns as the windings 6 the flux within the coil must be null balanced or of zero value, whatever may be the current value in the TM conductor.

The vectorial sum of the fluxes induced by the windings 6 and by the winding 7 in the case of a leakage grounding current of an intensity equal to i is equal to (N -N) i, whatever may be the intensity of the utilization current passing through the two windings 6. Now, the switch 12 should be opened only when the intensity i' rises above the value Idc tolerated for the normal leakage current passing through the grounding line, said value being a multiple of Idh.

But, on the other 'hand, the relay is actuated when the flux in the transformer is equal to NIdh; it is therefore necessary in the construction of the transformer to satisfy the relation: (N N)Idc=NIdh or N I do N1-N I dh Idc and Idh being predetermined values and N being selected in accordance with the desired sensitivity, it is possible to define the number of turns N to be given to the winding 7 in order that the relay may be actuated for any current flowing through the grounding line, the intensity of which is above Idh.

It is assumed that Idc is equal to 200 milliamperes and Idh is equal to 40 milliamperes, in which case N +%N, so that, for instance, N is equal to 30 turns N to 36 turns.

Considering now the control of the potential of the neutral with reference to ground, said potential arises generally inside the feed system and produces a current flowing towards ground through the resistance 16 and the primary winding 6 of the phase line or through the auxiliary primary winding 7, in the grounding line TM. When the intensity of said current is equal to Idh, the switch opens in the manner already described. If U is the maximum potential allowable for the neutral, the value r of the resistance 16 should be such that If the maximum potential U is equal to 22 volts, the following equation may be written:

ohms

which means that r is equal to 500 ohms if RM is equal to 50 ohms.

Conversely, if the resistance value of the grounding resistance RM is increased, the potential appearing inside the frame of the apparatus 2, as a consequence of normal supply current loss due to contact therein with water or steam, will increase and produce a current flowing towards the neutral through the auxiliary primary winding 6 7, the resistance 16 and the primary winding 6 of the neutral. The intensity of said current is a function of the potential Uc or contact voltage between the frame and the neutral, of the value r of the resistance 16 and of the grounding resistance RM.

If RM becomes infinitely large, the intensity i" of the current between the frame and the neutral is equal to Said current i" flows both through the auxiliary primary winding 7 and the primary winding 6 of the neutral and it acts consequently as a leakage current passing through the grounding line without producing however any lack of equilibrium between the phase line and the neutral. The lack of equilibrium produced by the difference of fluxes is equal to N i" or since the grounding resistance RN is negligible and the relay is actuated when said lack of equilibrium reaches a value NIdh.

Consequently, the switch opens for a contact voltage Uc equal to N IdhX T or /6 X 0.04 500=l6.6 volts.

If the value of the resistance RM is not infinite, the contact voltage produces a current i passing through the resistance RM. Consequently a flux Ni in the primary phase winding and a flux N i of an opposite direction in the primary ground winding are produced. Said contact voltage produces furthermore a current i" passing through the resistance r at 16 towards the neutral and producing a flux Ni in the primary neutral winding and NJ" in the primary grounding winding.

The flux passing through the secondary is therefore equal to (N N)i+N i. Said value being limited by a maximum NIdh, this leads to:

U61 RM and this shows that the value of the grounding resistance is always defined by the relationship:

1 U01 6 UC1 5 RM 5 7.7 dh The value of the contacting voltage actuating the relay for a grounding resistance equal to ohms and a resistance r equal to 500 ohms is therefore equal to:

ion 6Uc1 5100 552E or Uc =9 volts.

In other words, the first primary winding 6 and the second primary winding 6, as shown in the drawing, are so arranged on the transformer coil 5 that their fluxes are a resultant null during safe operation of installed appliance 2, and the auxiliary primary winding 7 is so arranged With respect to the first and second primary windings that during unsafe operation of the grounded appliance a resultant flux is induced by transient energized auxiliary primary win-ding 7 to energize secondary winding 8 to operate relay 10 to disconnect the appliance 2 from feeder lines, Ph and N.

The embodiment described hereinabove by way of example may obviously be modified in any desired manner without unduly widening thereby the scope of the invention as defined in the accompany claims.

What we claim is:

1. An automatic breaker for the control of an electrical appliance installation adapted to operate by feeder lines including at least, a neutral line and a phase line of an electrical power source, said phase line and neutral to the metal frame of said electrical appliance and interconnected to one lead of said auxiliary primary winding having another lead connected in series with a ground lead connected by ground resistance means to a ground, said second primary winding being so arranged on said transformer core that their fluxes are a resultant null during safe operation of said appliance, said auxiliary primary winding being so arranged with respect to said first and second primary windings that during unsafe grounded operation of said appliance a resultant flux is induced by the transient energized auxiliary primary winding to energize said secondary winding to operate said relay to disconnect said appliance from said feeder lines.

2. An automatic breaker for the control of. an elec trical appliance installation adapted to operate by feeder lines including at least, a neutral line and a phase line of an electrical power source, said phase line and neutral lines each including relay contacts therein, a relay adapted to operate said contacts, a transformer comprising a core with an auxiliary primary winding and a plurality of primary windings including a first primary winding and a second primary winding, an appliance winding having one lead connected in series with said first primary winding connected to said neutral line and the other lead of the appliance winding connected in series with said second primary winding interconnected to said phase line,

said transformer comprising a secondary winding in series with the winding of said relay, a grounding line connected to the metal frame of said electrical appliance and interconnected to one lead of said auxiliary primary winding having another lead connected in series with a ground lead connected by ground resistance means to a ground, .a second predetermined resistance means connected to a predetermined point on the ground lead between said auxiliary primary winding and the ground resistance, said second resistance being further connected in series with said other lead of the appliance winding and said first primary winding, and said second primary winding being so arranged on said transformer core that their fluxes are a resultant null during safe operation of said appliance, said auxiliary primary winding is so arranged with respect to said first and second primary windings that during unsafe grounded operation of said appliance a resultant l'lux is induced by the transient energized auxiliary primary winding to energize said secondary winding to 0perate said relay to disconnect said appliance from said feeder lines.

3. An automatic breaker for the control of an electrical appliance installation as in claim 2, wherein said transformer comprises a primary windingfor each phase and said neutral lines, said primary windings each having the same number of turn-s and being inserted in series in said feeder lines.

4. An automatic breaker for the control of an electrical installation according to claim 2 in which the difference in the number of turns of a primary winding and said auxiliary primary winding is such as to produce a resultant flux in said transformer core to energize said secondary winding to operate said relay to disconnect an unsafe grounded appliance from said feeder lines.

References Cited by the Examiner UNITED STATES PATENTS 1,667,655 4/28 Clothier 317-27 1,776,130 9/30 Petch 317-27 FOREIGN PATENTS 114,770 2/42 Australia. 350,751 6/ 31 Great Britain. 429,503 5/3 5 Great Britain. 900,437 10/44 France. 1,053,639 5/ 59 Germany.

SAMUEL BERNSTEIN, Primary Examiner. 

1. AN AUTOMATIC BREAKER FOR THE CONTROL OF AN ELECTRICAL APPLIANCE INSTALLATION ADAPTED TO OPERATE BY FEEDER LINES INCLUDING AT LEAST, A NEUTRAL LINE AND A PHASE LINE OF AN ELECTRICAL POWER SOURCE, SAID PHASE LINE AND NEUTRAL LINES EACH INCLUDING RELAY CONTACTS THEREIN, A RELAY ADAPTED TO OPERATE SAID CONTACTS, A TRANSFORMER COMPRISING A CORE WITH AN AUXILIARY PRIMARY WINDING AND A PLURALITY OF PRIMARY WINDINGS INCLUDING A FIRST PRIMARY WINDING AND A SECOND PRIMARY WINDING, AN APPLIANCE WINDING HAVING ONE LEAD CONNECTED IN SERIES WITH SAID FIRST PRIMARY WINDING CONNECTED TO SAID NEUTRAL LINE AND THE OTHER LEAD OF THE APPLIANCE WINDING CONNECTED IN SERIES WITH SAID SECOND PRIMARY WINDING INTERCONNECTED TO SAID PHASE LINE, SAID TRANSFORMER COMPRISING A SECONDARY WINDING IN SERIES WITH THE WINDING OF SAID RELAY, A GROUNDING LINE CONNECTED TO THE METAL FRAME OF SAID ELECTRICAL APPLIANCE AND INTERCONNECTED TO ONE LEAD OF SAID AUXILIARY PRIMARY WINDING HAVING ANOTHER LEAD CONNECTED IN SERIES WITH A GROUND LEAD CONNECTED BY GROUND RESISTANCE MEANS TO A GROUND, SAID SECOND PRIMARY WINDING BEING SO ARRANGED ON SAID TRANSFORMER CORE THAT THEIR FLUXES ARE A RESULTANT NULL DURING SFE OPERATION OF SAID APPLIANCE, SIAD AUXILIARY PRIMARY WINDING BEIG SO ARRANGED WITH RESPECT TO SAID FIRST AND SECOND PRIMARY WINDINGS THAT DURING UNSAFE GROUNDED OPERATION OF SAID APPLIANCE A RESULTANT FLUX IS INDUCED BY THE TRANSIENT ENERGIZED AUXILIARY PRIMARY WINDING TO ENERGIZE SAID SECONDARY WINDING TO OPERATE SAID RELAY TO DISCONNECT SAID APPLIANCE FROM SAID FEEDER LINES. 